1. Field of the Invention
The invention relates to a process for preparing alkyl tert-butyl ethers and butene oligomers from a mixture containing isobutane and n-butane, where the isobutane is converted to the alkyl tert-butyl ether and the n-butane is converted to the oligomers. The ratio of the two reaction products may be controlled by adjusting the ratio of n-butane to isobutane.
2. Description of the Background
Alkyl tert-butyl ethers (RTBE, where R represents alkyl) are used as additives to motor gasoline to increase the octane rating. They are generally prepared by addition of alkanols to isobutene, i.e., etherification. The isobutene may originate from four different sources: steam crackers, propylene oxide plants, petroleum refineries (i.e., FC crackers) and plants for the dehydrogenation of isobutane (cf. R. A. Pogliano et al., Dehydrogenation-Based Ether Production--Adding Value to LPG and Gas Condensate. 1996 Petrochemical Review, DeWift & Company, Houston, Tex.). In the first three sources, the isobutene arises as a constituent of the C.sub.4 fraction, that is as a direct byproduct. In the dehydrogenation of isobutane, isobutene is frequently a secondary byproduct of such plants, since the starting material isobutane is likewise obtained as a direct byproduct in steam crackers and petroleum refineries or by isomerization of n-butane, which itself is a byproduct in steam crackers and petroleum refineries. The current world production of RTBE is around 25 million metric ten/year, with an increasing trend. The production of butanes and butenes as byproducts in a particular cracker or a particular petroleum refinery is too small to be able to exploit completely the "economies of scale", which are latent in the RTBE process. Therefore, isobutene and/or isobutane (for dehydrogenation) would have to be collected from crackers and/or refineries, in order to be able to operate an RTBE plant at optimum capacity. Alternatively, sufficient C.sub.4 fraction could be collected from such plants and these could be worked up together on site to isobutene and isobutane. However, opposing both variants, and in particular the second, is the fact that the transport of liquid gases is expensive, in part due to the complex safety precautions necessary.
The term dibutene refers to the isomeric mixture which, in addition to higher butene oligomers, is formed by dimerization and/or codimerization of butenes, i.e., of n-butene and/or isobutene, in the oligomerization of butenes. Generally, the term dibutene refers to the dimerization products obtained from a mixture of n-butene and isobutene. The term di-n-butene refers to the dimerization product of n-butene, i.e., 1-butene and/or 2-butene. Significant components of the di-n-butene are 3-methyl-2-heptene, 3,4-dimethyl-2-hexene, and, to a minor extent, n-octenes. Di-isobutene is the isomeric mixture which is formed by dimerization of isobutene. Di-isobutene is more highly branched than dibutene and this, in turn, is more highly branched than di-n-butene.
Dibutene, di-n-butene and di-isobutene are starting materials for preparing isomeric nonanols by hydroformylation and hydrogenation of the C.sub.9 aldehydes thus formed. Esters of these nonanols, in particular the phthalic esters, are plasticizers which are prepared in large quantities and are primarily used for poly(vinyl chloride). Nonanols from di-n-butene are linear to a greater extent than nonanols from dibutene, which in turn are less branched than nonanols from di-isobutene. Esters of nonanols from di-n-butene have application advantages over esters from other nonanols and are, therefore, particularly in demand.
n-Butene is obtained for the dimerization, just as is isobutene, from C.sub.4 fractions, for example, that arise in steam crackers or FC crackers. The C.sub.4 fractions are generally worked up by first separating off 1,3-butadiene by a selective scrubbing, e.g., with N-methylpyrrolidone. Isobutene is a desirable and particularly valuable component of the C.sub.4 fraction because it may be chemically reacted, alone or in a mixture with other C.sub.4 hydrocarbons, to give sought-after products, e.g., with isobutene to give high-octane isooctane, or with an alkanol to give an RTBE, in particular with methanol to give methyl tert-butyl other (MTBE). After the reaction of the isobutene, the n-butenes and n-butane and isobutane remain behind. However, the proportion of n-butene in the cracked products of the steam crackers or the petroleum refineries is relatively small. In the case of steam crackers it is in the order of magnitude of barely 10 percent by weight, based on the principal target product ethylene. A steam cracker having the respectable capacity of 600,000 metric t/year of ethylene therefore only delivers aroung 60,000 metric t/year of n-butene. Although this amount (and that of the isobutenes) could be increased by dehydrogenating the approximately 15,000 metric t/year of n-butane and isobutane, which arise in addition to the n-butenes, this is not advisable, because dehydrogenation plants require high capital expenditure and are thus uneconomic for such a small capacity.
Isobutene is, as discussed above, a sought-after cracking product and is therefore generally not available for the isomerization to n-butene. The amount of n-butenes which a steam cracker or petroleum refinery produces directly is not sufficient, however, to produce sufficient di-n-butene for a nonanol plant of a high enough capacity that it could compete economically with the existing large-scale plants for preparing important plasticizer alcohols, such as 2-ethylhexanol. Propylene oxide plants are, as already stated even less productive still. n-Butenes would therefore have to be collected from various steam crackers, refineries or propylene oxide plants (or C.sub.4 fraction from various sources worked up to n-butene) and the combined n-butene oligomerized in order to cover the dibutene requirement of a sufficiently large economical nonanol plant. However, the transport of liquid gases is expensive and dangerous, as discussed above.
It would therefore be desirable if n-butene and isobutene could be provided at only one site without transport over relatively large distances in amounts as are required in a coupled production for the operation of a large economically advantageous plant for the preparation of di-n-butene, for example having a capacity of 200,000 to 800,000 metric t/year, and the same type of plant for preparing RTBE, e.g., having a capacity of 300,000 to 800,000 metric t/year. It would further be desirable to arrange the link between these plants in such a manner that the ratio of n-butene to isobutene can be set in accordance with the desired amounts of RTBE and butene oligomers.